The present invention relates generally to free-piston devices, and, more particularly, to free-piston devices having reciprocating pistons with drift stabilization.
Free-piston power conversion devices (engine-generators, gas compressors, and closed-cycle cooling machines) have been long known. Typically, such machines incorporate at least one internal, reciprocating piston that seals the internal volume into two spaces. The piston reciprocates, typically at resonant conditions, with a pressure wave generated by the change of volume during reciprocation in the two spaces sealed apart by the piston. Because one great advantage of such devices is their ability to operate fully sealed for long periods, and especially in engine applications where part of the system may be held at high temperature or in refrigerator applications where part of the system may be held at low temperature, no lubricant is provided to ease the relative motion of the piston and stationary parts. Indeed, the desirable absence of lubricant is the reason for the development of such resonant xe2x80x9cfreexe2x80x9d pistons, the motion of which is determined not by linkages, but by the sum of forces from gas pressure and electromagnetic fields.
Means to achieve a tight seal between the piston and the fixed parts (typically a cylinder around the piston) are limited to methods that do not demand lubrication. Most commonly, this is met by use of a close clearance fit between the piston body and the cylinder, which impedes the flow past the piston to a near-negligible fraction of the piston""s displaced volume. This leakage flow is ideally a fully-reversing flow, with no net mean flow in either direction. However, small variances in the shaping of the clearance gap, such as tapers, rounded entries, etc. can produce a tendency for flow to be preferred in one direction over the other. This leakage can lead to an excess of fluid on one side of the piston. If the piston were restrained in its motion by a linkage, this would appear as increased pressure in the space with excess fluid, with that pressure rising above the pressure of the other space until the differences would drive a reverse flow equal to the leakage, producing a stable zero net flow. In a free piston, though, such a build-up of steady pressure difference across the piston is not sustainable because there is no linkage to hold the piston in position and force the space volumes to be unchanged. Instead, the piston moves, in a steady motion superimposed on its reciprocation, toward the space losing fluid and away from the space accumulating fluid via the seal leakage. This is called xe2x80x98driftxe2x80x99 and can prevent the stable operation of any free-piston machine.
All free-piston systems known to date provide some means to control drift. These include: strong axial springs to provide some of the position fixation a linkage would, but still allowing reciprocation; centerports, which short-circuit the piston seal momentarily in every reciprocation, ideally at a time when the pressures in the spaces ought to be equal (and generating a corrective flow only if they are not); and active controls that sense piston position and operate discrete valves to pump fluid back in opposition to leakage. Axial springs are expensive, high-stress components that impose unwanted secondary (non-axial) forces that impair reliability. Centerports typically cannot be located precisely where the pressures are exactly equal, causing a flow even when not required for drift control. Thus, centerports exact a significant efficiency penalty, and, indeed, are not usable at all in some classes of machines that have large temporal phase differences between piston motion and pressure oscillations (e.g. standing-wave thermoacoustics). Active controls are very expensive and can significantly diminish the reliability, safety, and simplicity of free-piston machines.
The present invention provides a means to passively and automatically correct for the steady leakage effect underlying piston drift without large cost, friction, reliability reduction, or significant efficiency loss.
Various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention is directed to a free-piston device where piston drift is stabilized. A piston having a frequency of reciprocation over a stroke length and with first and second sides facing first and second variable volumes, respectively, for containing a working fluid defining an acoustic wavelength at the frequency of reciprocation. A bypass tube waveguide connects the first and second variable volumes at all times during reciprocation of the piston. The waveguide has a relatively low impedance for steady flow and a relatively high impedance for oscillating flow at the frequency of reciprocation of the piston, so that steady flow returns fluid leakage from about the piston between the first and second volumes while oscillating flow is not diverted through the waveguide. Thus, net leakage about the piston is returned during each stroke of the piston while oscillating leakage is not allowed, and pressure buildup on either the first or second side of the piston is avoided to provide a stable piston location.